1. Field of the Invention
The invention relates to a method of implementing dynamic channel allocation in a cellular radio system, the system comprising: in each cell, at least one base station which communicates with subscriber terminals located within its area, which broadcasts at least at one carrier frequency and which transmits information about itself on control channels to subscriber terminals; in at least one cell, at least two subcells which comprise an antenna unit and in which all available frequencies are dynamically used; and a base station of the system measuring, through each antenna unit, a level of interference of all available traffic channels, on the basis of which that traffic channel in an antenna unit selected for connection set-up which has the least interference is selected for communication.
2. Description of the Related Art
In cellular radio systems, a user""s speech and data between a base station and a subscriber terminal are transmitted on a traffic channel. Between a base station and a subscriber terminal are also needed various control messages and system information, which are transmitted on control channels. An example of a control channel that can be mentioned is a BCCH channel, which is used in the GSM system for transmitting connection set-up information from a base station to subscriber terminals.
In the current GSM system, a carrier frequency that comprises a BCCH channel, i.e. a BCCH carrier frequency, is transmitted uninterruptedly at a constant power level. A subscriber terminal continuously measures the power level of the BCCH carrier frequencies transmitted by adjacent base stations and reports the measurement results to the base station serving the subscriber terminal. On the basis of the measurement results, the system decides an appropriate moment for a handover to another base station.
The requirement that in the current GSM system a BCCH carrier frequency is to be transmitted uninterruptedly at a constant power level prevents the use of certain methods developed to improve connection quality and system capacity on control and traffic channels of a BCCH carrier frequency. The methods developed to improve connection quality and system capacity in cellular radio systems include for instance frequency hopping, discontinuous transmission and adjustment of transmission power.
Effective utilization of a frequency spectrum is one of the main objectives in cellular radio systems. Most channel allocation methods are based on the reuse of frequencies beyond a given interference distance. The conventional GSM system is implemented by using Fixed Channel Allocation (FCA). In this method the frequencies available for use in the system are divided into groups and fixedly allocated to different cells. The frequency groups can be reused in cells which are at a sufficient distance from each other. The reuse distance is determined by the level of co-channel interference that the system tolerates.
If the traffic load estimate concerning each cell is correct and the traffic loads do not vary greatly, fixed channel allocation functions reasonably well in macrocells, i.e. in cells the size of which is between one kilometer and several dozens of kilometers. A disadvantage in fixed channel allocation is that precise and laborious frequency planning is required to allow co-channel interference to be minimised. In addition, fixed channel allocation adjusts poorly to variations in traffic loads.
The above disadvantages in fixed channel allocation are emphasised in cellular radio systems with a small cell size, such as office systems, where variations in radio traffic load are usually larger than in macrocell systems, which complicates radio network planning. The load of radio traffic grows strongly in environments using small cells, and the network should be able to easily adapt to the increase in traffic. The adding of new cells to a network applying fixed channel allocation requires, however, that frequency planning as a whole is renewed.
To obtain a solution to the above-mentioned problems, a more flexible and more adjustable channel allocation method than the one provided by fixed channel allocation is required. In the purest form of Dynamic Channel Allocation (DCA), all frequencies of the system are available in every cell. No frequency group is allocated to a particular base station; instead, all channels can be taken in use at any base station. Channel selection can be made at the moment of call set-up on the basis of the interference situation at the time concerned. The major advantages of dynamic channel allocation are its flexibility in connection with different traffic loads and its greater efficiency in the utilization of the frequency spectrum. In addition, frequency planning is no longer needed. As cell size diminishes, the advantages of and the need for dynamic channel allocation become further emphasised.
An intermediate form of fixed and dynamic channel allocation is a channel allocation method in which some of the channels are fixedly allocated and others are dynamically available. Also in such a method the advantages offered by dynamic channel allocation can be utilized.
An object of the present invention is thus to provide dynamic channel allocation in a cellular radio system.
This is achieved with a method described in the preamble, characterized in that all antenna units listen to control channels used by subscriber terminals for call set-up, and a base station measures the power level of the control channels through antenna units, on the basis of which an antenna unit providing the strongest signal is selected for connection set-up; and characterized in that common control channels are transmitted through all antenna units; and characterized in that traffic channels are transmitted through an antenna unit in which a signal received from a subscriber terminal is the strongest; and characterized in that a traffic channel is reused in another subcell.
The method of the invention provides distinct advantages compared with the prior art. The invention enables dynamic channel allocation to be performed in a cellular radio system. This improves the flexibility of the system in connection with different traffic loads and enhances the utilization of the frequency spectrum. Also the need for frequency planning is substantially reduced. The method of the invention thus enables a reuse of traffic channels in different subcells within a cell. The advantages of dynamic channel allocation are particularly emphasised in networks with small cell sizes.